Source label maps

[ndilalla]

Implemented the feature to label only a single source (or a list of sources) in TS and residual maps (issue #623 )

• Added label_source parameter that accepts a string (single source) or a list of source names
• When label_source is provided, only those sources are labeled, overriding the label_ts_threshold logic
• If label_source is not specified, the existing TS threshold behavior is used

[codecov]

Codecov Report

❌ Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.80%. Comparing base (92cf94f) to head (ff8b3cc).

Files with missing lines	Patch %	Lines
fermipy/plotting.py	72.72%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #657   +/-   ##
=======================================
  Coverage   50.79%   50.80%           
=======================================
  Files         143      143           
  Lines       27265    27273    +8     
=======================================
+ Hits        13849    13855    +6     
- Misses      13416    13418    +2     

Flag	Coverage Δ
unittests	50.80% <72.72%> (+<0.01%)	⬆️

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